1. Field of the Invention
This invention relates generally to sputtering apparatus using a magnetic field. In particular, this invention is applicable to sputtering operations wherein the geometrical relationship of workpiece positions to a cathode results in some of the workpieces becoming coated with different thicknesses of material than others. The present invention is advantageous where nonuniformity in the thicknesses of material deposition between workpieces of the same lot are not desirable.
2. Discussion of the Prior Art
Sputtering methods to deposit material on substrates are commonly used in the electronics or semiconductor industry where thin metal coatings or films are formed on substrates or semiconductor devices as part of processes for producing conductive contacts and interconnections. For example, uniform metal films may be deposited over the entire surfaces of semiconductor wafers of arrayed, partially finished integrated-circuit devices. Later the surfaces are selectively masked and etched to form contact patterns for these devices.
A suitable type of commercial sputtering apparatus for depositing films of material on articles such as semiconductor wafers employs an evacuable enclosure or chamber. Within the chamber a circular cathode, commonly referred to as the target, is located. An anode is located in a parallel, spaced relationship to the target. A workholder or pallet is mounted to the anode facing the target. The pallet holds a predetermined number of workpieces, or in the case of semiconductor devices, wafers. The number of wafers placed onto the pallet for each cycle of operation corresponds to the largest number of wafers which can be processed by the apparatus in a single batch or lot.
It is, of course, desirable in sputtering films onto the wafers that the thickness of the film on anyone of the wafers is substantially equal to that of the film of any other of the wafers. Rotating the anode with the pallet and wafers thereon during sputtering along an axis perpendicular to the plane of the anode and the target tends to smoothen out variations in thickness relating to angular differences in the sputtering action of the apparatus.
However, it is known that the rate of material deposition tends to diminish toward the outer perimeter of the pallet. Such a decrease in material deposition at the periphery of the pallet is logically explainable. While a circular area opposite a centrally located point on the pallet falls fully on the target, a similarly circular area opposite a point on the periphery of the pallet lies partially outside the periphery of the target. Consequently, a peripheral point on the pallet tends to have less exposure to material eroding from the target.
To minimize nonuniformity of sputtered films caused by such decreased material deposition in peripheral areas, prior art sputtering techniques involve the use of targets which are larger in area than the corresponding pallets. However, for any given apparatus, the size of the target is limited, and such a limitation necessitates a corresponding reduction in the size of the pallet. The size of the pallet on the surface of which a desirable uniformity of material deposition can be achieved, therefore, tends to be dictated by the size of the target.